1. Field of the Invention
The present invention relates to a rechargeable lithium battery, and more particularly, to an improved rechargeable lithium battery having excellent reliability and cycle-life characteristics attainable by inhibiting volume expansion caused by high-capacity negative active materials.
2. Description of the Related Art
Batteries include primary batteries which can be used only once and are then disposed of and secondary batteries which can be recharged and used repeatedly. Primary batteries include manganese batteries, alkaline batteries, mercury batteries, silver oxide batteries, and the like. Secondary batteries include lead-acid storage batteries, nickel-metal hydride (Ni—MH) batteries, sealed nickel-cadmium batteries, lithium metal batteries, lithium ion batteries, lithium polymer batteries, lithium-sulfur batteries, and the like.
A lithium battery generally includes a negative electrode and a positive electrode, and a separator disposed between the negative electrode and the positive electrode. The negative electrode provides electrons during a battery discharge, a positive electrode gains electrons provided by the negative electrode during the battery discharge, the separator sandwiched between said positive electrode electrically insulates the negative electrode and the positive electrode. Lithium batteries generally use lithium metal or lithium compounds to form electrodes.
Recently, lithium batteries have been required to have high capacity. Since portable electronic devices are widely used and generally use the lithium batteries as power source, the lithium batteries are required to have lighter weight and better performance in compliance with the current market requirements. Meanwhile, research and development on active materials have been active in these years. Lithium metal has been contemporarily used as a negative active material, however, the lithium metal may form dendrites and thereby may cause a battery short-circuit. Such battery short-circuit may cause an explosion of a battery having the lithium metal.
A carbon-based material instead of the lithium metal is now mostly used as a negative active material. This carbon-based active material used as a negative active material forming a lithium battery may include crystalline carbon such as graphite and artificial graphite, or amorphous carbon such as hard carbon. Even though the amorphous carbon has large capacity, a problem of substantial non-reversibility occurs during charge and discharge. The crystalline carbon representatively includes graphite. The graphite has high capacity because the graphite has a theoretical capacity limit of 372 mAh/g, however, the graphite has severely deteriorated cycle-life. Even though the graphite or carbon-based active material has comparatively high theoretical capacity (a limit of 372 mAh/g), the graphite or carbon-based active material cannot be used for the aforementioned negative electrode of a desirable high-capacity lithium battery because the theoretical capacity of the graphite or carbon-based active material is not higher than 380 mAh/g.
In order to solve this problem, active research focuses on a metal-based or inter-metallic compound which may be used as a negative active material. For example, metals or semi-metals such as aluminum, germanium, silicon, tin, zinc, lead, and the like have been determined as candidates of a negative active material. These materials have high capacity and high energy density. Since they can intercalate and deintercalate more lithium ions than a carbon-based negative active material, they can contribute to a lithium battery with high capacity and high energy density.
For example, pure silicon is known to have a high theoretical capacity of 4017 mAh/g. The pure silicon however has deteriorated cycle characteristics compared with a carbon-based material, therefore, the pure silicon has not yet been successfully used to manufacture of the lithium battery. The reason of failing to use silicon in forming the lithium battery is that, a mineral particle such as silicon or tin may have change of volume during charge and discharge of the battery, and thereby may deteriorate conductivity or may be delaminated from a negative electrode current collector. For example, when lithium ions is intercalated during the charge, the aforementioned mineral particle such as silicon or tin included in a negative active material may expand up to approximately from 300 to 400% in volume. When the lithium ions are deintercalated during the discharge, the mineral particles contract and thereby form a space departed from the active material. The space may cause an electrical insulation which severely deteriorates cycle-life of a battery.
Accordingly, Japanese Patent Laid-Open Publication No. 2005-71655 discloses a method of plating copper on a silicon surface and forming an alloy. However, the method is extremely tedious and complex and includes many processes such that the process may not be efficient in terms of economy. An amorphous alloy oxide has been suggested as a negative active material to solve the above problems (Y. Idota, et al.: Science, 276, 1395 1997). In addition, an amorphous alloy has been disclosed as a negative active material in the 43rd Battery Forum Preview Collection (Corporate Electrochemical Battery Technology Committee, Pyeung 14, October 12th, from page 308 to page 309). While silicon is known as an element that can be expected to have high capacity, however, it is difficult for silicon to exist in an amorphous form by itself alone. It is also difficult for a silicon alloy to exist in an amorphous form. According to a recently reported mechanical alloy, however, a silicon-based material may easily become amorphous. Even though the amorphous silicon alloy material has high initial cycle capacity retention compared with a crystalline alloy material, the silicon alloy material tends to have easily deteriorated capacity retention. The amorphous silicon alloy material also has a lower expansion rate and is less likely to deteriorate during charge and discharge compared to a crystalline material, since the amorphous silicon alloy material does not have a singular structure. (43rd Battery Forum Preview Collection (Corporate Electrochemical Battery Technology Committee, Pyeung 14, October 12th, from page 308 to page 309)) In addition, the amorphous silicon alloy material can be prepared to be amorphous or microcrystalline through repeated grinding and compressing and then assembly, while gradually decreasing the crystallinity of the amorphous silicon alloy material, for example, by a mechanical alloying method. However, the amorphous silicon alloy material is excessively broken at the interface among alloy structures and is broken down and pulverized due to intercalation and deintercalation of lithium ions during the charge and discharge, and thus the amorphous silicon alloy material cannot even be distinguished in an X-ray diffraction analysis, resultantly deteriorating cycle-life.
Because none of existing active materials are desirable for the formation of the lithium batteries, a novel active material having high capacity and an improved capacity retention characteristic and a lithium battery including the active material and having an improved cycle-life characteristic are required.